This invention relates to an internal combustion engine with a combustion chamber and an exhaust gas accumulation chamber, more particularly to an internal combustion engine wherein a part of a combustible gas mixture after being burned in the combustion chamber is temporally accumulated within the accumulation chamber, and then the accumulated burnt gas is mixed with the next charge of combustible intake gas introduced into the combustion chamber, so that oxides of nitrogen (NO.sub.x) in the exhaust gas emitted from the engine can be reduced.